378 A. C. LAXE STUDIES OF GKAIX OF IGXEOUS IXTEUSIVES 



and this continues clear to the center. At about the same distance from 

 the margin (600 millimeters) at which the porphyritic texture ceases we 

 commence to find the interstices of micro-pegmatite, with the associated 

 biotite, apatite, and hornblende, which I have described at such length 

 in my Isle Royal report. As we proceed toward the center of the dike, 

 they become coarser, better marked, and the apatite in them, I think, a 

 shade larger. This increasing coarseness of grain, however, I do not 

 take to be a mere matter of cooling, but due to the fact that in the pro- 

 cess of consolidation of the dike it solidified first at the side, and then 

 the more soluble residue represented by these interstices was squeezed 

 toward the center. 



When we come to study the magnetite, we find that increasing from 

 nothing or an imperceptibly fine dust at the margin clear and continu- 

 ously to the center, and for this, as well as the feldspar, we are constrained 

 to believe that the formation took place in the very earliest stage of 

 cooling. 



The olivine is exceedingly interesting. Sharply defined phenocrysts 

 of olivine are visible right up to the ver}^ margin, and the}" increase in 

 size somewhat for a while as we shift our point of view farther from the 

 margin. This shows that though like the feldspar, the}^ may have com- 

 menced to form before the dike came to rest, they were not ver}^ large 

 then and increased and continued to grow afterward, but as we continue 

 toward the center we find that they do not continue to grow, or rather 

 that we can not depend on it. We will find occasional!}^ quite a large 

 grain, but our curve of average grain may not increase at all. This does 

 not, however, mean that the olivine belongs to the same period of forma- 

 tion of the augite, for a study of the grains shows that while at the 

 margin they have sharp well defined forms, such as olivine crystals 

 have, at the center they are very likely to be in the irregular corroded 

 grains and are much rarer, anyway. A study of the slides makes it per- 

 fectly plain that Avhat has happened is this : The olivine did crystallize 

 out quite early, yet for the most part not before the dike came to rest. 

 It was probably quite coarse at the center. After it had formed, how- 

 ever, the concentration of the salic residue, which we have already de- 

 scribed, altered the chemical composition of the magma at the center, 

 and it was corroded. Probably some of it went to form augite, but part 

 of it went into the ])iotite. 



Franklin Furnace Minette 



Queneau gives quite a complete study of the grain of the biotite and 

 apatite of the minette dike at Franklin Furnace, New Jersey, the width 

 of which is .61 millimeters. From the tables of grain of the biotite it is 



